Apparatus for polishing thin, flat semi-conductor wafers is well-known in the art. Such apparatus normally includes a polishing head which carries a membrane for engaging and forcing a semi-conductor wafer against a wetted polishing surface, such as a polishing pad. Either the pad, or the polishing head is rotated and oscillates the wafer over the polishing surface. The polishing head is forced downwardly onto the polishing surface by a pressurized air system or, similar arrangement. The downward force pressing the polishing head against the polishing surface can be adjusted as desired. The polishing head is typically mounted on an elongated pivoting carrier arm, which can move the pressure head between several operative positions. In one operative position, the carrier arm positions a wafer mounted on the pressure head in contact with the polishing pad. In order to remove the wafer from contact with the polishing surface, the carrier arm is first pivoted upwardly to lift the pressure head and wafer from the polishing surface. The carrier arm is then pivoted laterally to move the pressure head and wafer carried by the pressure head to an auxiliary wafer processing station. The auxiliary processing station may include, for example, a station for cleaning the wafer and/or polishing head; a wafer unload station; or, a wafer load station.
More recently, chemical-mechanical polishing (CMP) apparatus has been employed in combination with a pneumatically actuated polishing head. CMP apparatus is used primarily for polishing the front face or device side of a semi-conductor wafer during the fabrication of semi-conductor devices on the wafer. A wafer is "planarized" or smoothed one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer is polished by being placed on a carrier and pressed face down onto a polishing pad covered with a slurry of colloidal silica or alumina in de-ionized water.
A schematic of a typical CMP apparatus is shown in FIGS. 1A and 1B. The apparatus 10 for chemical mechanical polishing consists of a rotating wafer holder 14 that holds the wafer 10, the appropriate slurry 24, and a polishing pad 12 which is normally mounted to a rotating table 26 by adhesive means. The polishing pad 12 is applied to the wafer surface 22 at a specific pressure. The chemical mechanical polishing method can be used to provide a planar surface on dielectric layers, on deep and shallow trenches that are filled with polysilicon or oxide, and on various metal films. CMP polishing results from a combination of chemical and mechanical effects. A possible mechanism for the CMP process involves the formation of a chemically altered layer at the surface of the material being polished. The layer is mechanically removed from the underlying bulk material. An altered layer is then regrown on the surface while the process is repeated again. For instance, in metal polishing a metal oxide may be formed and removed repeatedly.
A polishing pad is typically constructed in two layers overlying a platen with the resilient layer as the outer layer of the pad. The layers are typically made of polyurethane and may include a filler for controlling the dimensional stability of the layers. The polishing pad is usually several times the diameter of a wafer and the wafer is kept off-center on the pad to prevent polishing a non-planar surface onto the wafer. The wafer is also rotated to prevent polishing a taper into the wafer. Although the axis of rotation of the wafer and the axis of rotation of the pad are not collinear, the axes must be parallel. Polishing heads of the type described above used in the CMP process are shown in U.S. Pat. Nos. 4,141,180 to Gill, Jr., et al.; 5,205,082 to Shendon et al; and, 5,643,061 to Jackson, et al. It is known in the art that uniformity in wafer polishing is a function of pressure, velocity and the concentration of chemicals. Edge exclusion is caused, in part, by non-uniform pressure on a wafer. The problem is reduced somewhat through the use of a retaining ring which engages the polishing pad, as shown in the Shendon et al patent.
Referring now to FIG. 1C, wherein an improved CMP head, sometimes referred to as a Titan.RTM. head which differs from conventional CMP heads in two major respects is shown. First, the Titan.RTM. head employs a compliant wafer carrier and second, it utilizes a mechanical linkage (not shown) to constrain tilting of the head, thereby maintaining planarity relative to a polishing pad 12, which in turn allows the head to achieve more uniform flatness of the wafer during polishing. The wafer 10 has one entire face thereof engaged by a flexible membrane 16, which biases the opposite face of the wafer 10 into face-to-face engagement with the polishing pad 12. The polishing head and/or pad 12 are moved relative to each other, in a motion to effect polishing of the wafer 10. The polishing head includes an outer retaining ring 14 surrounding the membrane 16, which also engages the polishing pad 12 and functions to hold the head in a steady, desired position during the polishing process. As shown in FIG. 1C, both the retaining ring 14 and the membrane 16 are urged downwardly toward the polishing pad 12 by a linear force indicated by the numeral 18 which is effected through a pneumatic system.
In the polishing operation shown in the enlarged cross-sectional view of FIG. 1B, the slurry solution 24 must be forced into an interface between the wafer 10 and the polishing pad 12 in order for the chemical reaction and the mechanical removal process to operate efficiently. The slurry solution 24 (also shown in FIG. 1A) is dispensed from a dispensing nozzle (shown in FIG. 2) onto the polishing pad 12. In most commercial CMP apparatus, the slurry solution 24 is stored in a reservoir and delivered to the dispensing nozzle through a conduit. The slurry solution stored in the reservoir and in the delivering conduit is not provided with a temperature control device. The slurry solution 24 is normally applied to the polishing pad 12 at the same temperature as the chamber temperature in the CMP apparatus, i.e., approximately at room temperature.
During the polishing process, a significant amount of frictional heat is generated between the top surface of the polishing pad and the surface of the substrate that is being polished. The interface formed between the wafer 10 and the polishing pad 12 (shown in FIG. 1B) and the slurry solution 24 trapped therein are therefore heated to a significant higher temperature, i.e., up to 50.degree. C., or between about 40.degree. C. and about 50.degree. C., than the temperature of the slurry solution 24 on the edge of the wafer 10. This creates a serious problem in achieving polishing uniformity across the surface of the wafer 10. The room temperature slurry solution dispensed around the edge of the wafer 10 during the polishing process reduces the temperature of the wafer at the edge portion. This leads to a wafer edge polishing rate drop and a poor polishing uniformity.
It is therefore an object of the present invention to provide a method and apparatus for providing slurry temperature control in a polishing process that is not previously available in conventional polishing machines.
It is another object of the present invention to provide a method for slurry temperature control in a polishing process that can be readily adapted in a chemical mechanical polishing machine.
It is a further object of the present invention to provide a method for slurry temperature control in a chemical mechanical polishing process that allows the slurry to be heated before it is delivered to the polishing surface.
It is another further object of the present invention to provide a method for controlling slurry temperature in a chemical mechanical polishing process in which temperature sensors are utilized to detect the temperature of the dispensed slurry on the surface of the polishing head and the temperature of the polishing pad.
It is still another object of the present invention to provide a method for controlling slurry temperature in a chemical mechanical polishing process that utilizes two temperature sensors, a temperature controller and a heating apparatus.
It is yet another object of the present invention to provide an apparatus for controlling slurry temperature in a chemical mechanical polishing apparatus that can be easily adapted into a conventional CMP machine.
It is still another further object of the present invention to provide a chemical mechanical polishing method by utilizing temperature sensors, a temperature controller and heating devices to provide a heated slurry solution on a surface of the polishing pad for improved polishing uniformity.
It is yet another further object of the present invention to provide an apparatus for controlling slurry temperature in a chemical mechanical polishing machine that includes a polishing disc, a polishing head, motor means for rotating the disc in the polishing head, a slurry dispensing nozzle, a controller for comparing a first temperature of the slurry and a second temperature of the polishing pad, and a heater for heating the slurry to substantially the same temperature of the polishing pad in order to improve the polishing uniformity.